MELBOURNE, Australia — Angelique Kerber had already proved that she could handle the biggest moments in tennis. Madison Keys had not, and a one-sided women’s quarterfinal at the Australian Open on Wednesday did not resolve that disparity.

Kerber, who just turned 30, is back on a high in Melbourne: reading the flow of play beautifully, bending low and slapping straight-line winners off ill-considered shots from her opponents.

Even on a very good day for Keys, beating this version of Kerber would have been difficult. But on another tough day for Keys in the Grand Slam spotlight, this match quickly turned into a mismatch.

It was 4-0 Kerber in 14 minutes, 6-1 Kerber in 22 minutes. And, ultimately, despite all of Keys’s ball-striking talent and earnest off-season efforts, it was 6-1, 6-2, Kerber, in 51 minutes.

“I was just playing my game, not thinking a lot about winners or errors, just trying to stay in the moment and play every single point,” Kerber said.

It will be quite a surprise if her next round goes as quickly. In the semifinals on Thursday, she will face Simona Halep, the No. 1 seed who is, like Kerber, one of the supreme movers and counterpunchers in the women’s game.

“For sure is going to be the second marathon this tournament,” said Halep, who saved three match points before defeating the unseeded American Lauren Davis, 4-6, 6-4, 15-13, in the third round.

Halep had to right the ship again in the quarterfinals, falling behind 0-3 and facing a break point on her serve that would have made it 0-4 against the big-hitting Karolina Pliskova, a former No. 1.

But Halep managed to wriggle free and reeled off nine games in a row, focusing her attention on Pliskova’s less reliable forehand and on her own strengths.

Final score: 6-3, 6-2, Halep.

“Even in practice, I feel she just likes my game,” said Pliskova. “I think she’s just using my speed. That’s the problem. I try to play fast, but I think she likes this. She doesn’t have to give any power into her shots. She just uses my speed. Then in the end, I’m the one who is running. It doesn’t make sense.”

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Keys had nearly twice as many unforced errors as winners and often rushing her shots in the loss to Kerber.CreditSaeed Khan/Agence France-Presse — Getty Images

Pliskova said her cause was not helped by the late finish of her three-set, fourth-round victory over Barbora Strycova, which ended well after 1 a.m. on Tuesday.

“I went to sleep at seven in the morning yesterday,” Pliskova said. “I think just these late-night matches, they should cancel.”

Thursday’s other semifinal will match the unseeded Elise Mertens of Belgium against No. 2 seed Caroline Wozniacki of Denmark.

Of the semifinalists, only Kerber has won a major singles title. Halep and Wozniacki have each lost twice in Grand Slam finals, and both have saved multiple match points this year, with Wozniacki saving two in her second-round match against Jana Fett, coming back from 1-5 down in the third set.

Mertens, an unseeded 22-year-old from the overachieving tennis nation of Belgium, is playing in her first Australian Open.

Kerber’s expeditious victory over Keys should boost her already soaring confidence. She looks ready to run and run — a good idea against Halep — even if she joked on Wednesday that as a newly minted 30-year old, she needed to shorten the rallies.

“I am getting so old, so I have to be aggressive,” said Kerber, the No. 21 seed from Germany. “I cannot run for every ball anymore. I have to change something.”

She actually has changed plenty for 2018, splitting with her longtime coach Torben Beltz and hiring Wim Fissette, a veteran Belgian coach who has been a huge influence on other champions in the past, including Kim Clijsters, Victoria Azarenka and Halep.

Fissette does not tend to stay in his posts for long, but he is clearly an excellent analyst.

“She had great success with Torben but sometimes you need to change, just to hear a different voice,” said Barbara Rittner, the German Fed Cup captain. “It’s not a matter of the coach is bad or the player is bad. You just need to hear something new to respond better and try new things.

“Wim is a positive guy. He has a game plan, and he knows what he’s doing.”

The effects have been immediate. After winning four matches at the exhibition Hopman Cup team event in Perth, Australia, Kerber has won 10 straight official matches to start the season.

After winning the Australian Open and United States Open in her breakthrough 2016 season, she struggled for much of 2017. But Wednesday’s victory guaranteed that she would return to the top 10 after this tournament.

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Simona Halep after her win over Karolina Pliskova in the quarterfinals on Wednesday.CreditMast Irham/European Pressphoto Agency

“So many things happened to her, both good and bad, in the last 24 months,” Rittner said. “We talked about it in Luxembourg at the end of the year. It really was a lot to absorb, and she is someone who is thinking about things quite a lot, not just taking it lightly and going her way. It took her a while just to settle down and find herself again — having fun and being focused. And for the moment she is not looking back or looking forward.”

Facing Kerber did not look like much fun for Keys on Wednesday. Keys is hardly the only big hitter with limited defensive skills to suffer here at Kerber’s deft hands. She also routed Maria Sharapova, 6-1, 6-3, in the third round in 64 minutes before nearly falling against the stylistic outlier Hsieh Su-Wei in the fourth round, finally prevailing in three sets.

But Sharapova, despite the lopsided score, looked fully engaged in the combat from start to finish. Keys, despite some glimpses of her big-hitting potential, appeared to be edgy and restricted, much as she did in last year’s U.S. Open final against Sloane Stephens, when Keys also won just three games.

“I think she played really well,” Keys, the No. 17 seed, said of Kerber. “I felt like I was trying different things, but I wasn’t playing very consistent. I think in the first set I played really passive, and because of that I feel like I wasn’t moving as well, wasn’t accelerating.

“So I wasn’t very happy with how I played today, but still, I was fighting and trying to stay in the match. Sometimes you just don’t play very well out there.”

But Keys said she believed she and her coach, Lindsay Davenport, formerly an intimidating baseline player who won three major singles titles, are on the right track.

“I feel that,” Keys said. “I think how I played all of the other matches definitely shows that, and I’m not going to walk away from this tournament and think it was terrible because I had one bad match. I definitely think I have taken a lot of steps in the right direction and feel good about my game and feel like I’m thinking a lot clearer out there. I think it’s just going to take a little bit of time.”

In earlier years, a defeat like Wednesday’s might have left her tight-lipped and emotional, but Keys was expansive and dry-eyed in her post-match news conference.

“I’m evolving guys,” she said, smiling. “I’m growing.”

Keys can be extremely tough when she is serving big and swinging with confidence, as she showed against Caroline Garcia, the rising French star, in the fourth round here.

But she could not find a way to do justice to her abilities on Wednesday, making nearly twice as many unforced errors as winners and often rushing when patience would have been advisable.

Kerber is a tough matchup for Keys even on a good day. She has now lost seven of her eight matches against the counterpunching German, the last four in straight sets.

This was far, far from a good day.

A version of this article appears in print on , on Page B9 of the New York edition with the headline: Kerber, Playing in Top Form, Overwhelms an Underperforming Keys. Order Reprints | Today’s Paper | Subscribe